Fiber-optic current sensors commonly rely on the Faraday effect in an optical glass fiber. The fiber forms at least one loop around a conductor that carries the current to be measured. The Faraday effect is observed either as a rotation of the polarization of linearly polarized light or equivalently as differential phase shift between left and right circularly polarized light waves. Sometimes the two sensor versions are referred to as polarimetric and interferometric sensors, respectively. Advantageously, the fiber is operated in a reflective mode, i.e. the light performs a roundtrip through the fiber coil.
The magneto-optic polarization rotation or phase shift is converted to a change in light intensity by means of a polarizer. In order to get a linear variation of the light intensity as a function of current, it is necessary to introduce a 45° bias to the polarization angle, if the polarization rotation is detected, or a 90° phase bias, if the phase shift is measured. Frosio et al. [1] have applied the method of non-reciprocal phase modulation known from fiber gyroscopes [2] to dynamically generate a 90° phase bias by means of a phase modulator in interferometric sensors. Alternatively, the phase bias can be generated in a passive manner by means of a quarter wave retarder [1]. [3] describes a detection scheme with several detection channels that are anti-phase and/or at quadrature to each other also using passive retarders. The scheme allows compensation of certain imperfections such as temperature dependent signal bias and variation in the interference fringe visibility.
F. Brifford et al. [4] and K. Kurosawa et al. [5] describe a method where the 45° bias angle for linearly polarized light in polarimetric sensors is generated by a 22.5° in-line Faraday rotator. The light passes the rotator twice during its roundtrip through the sensor which results in the 45° bias rotation. In a similar arrangement, H. Lin et al. [6] use a 22.5° in-line Faraday rotator to introduce a 90° phase offset between left and right circular polarized light waves.
US patent application US2007/0273358A1 [7] teaches a method to compensate the temperature dependence of the Verdet constant in a sensor according to Ref. [5] by making use of the temperature dependence of the rotation angle of the Faraday rotator. US patent application US 2010/0253320 A1 [8] discloses a method to account up to first order for the temperature dependence of a sensor according to Ref. [5] by means of signal processing, or up to higher orders by means of signal processing and an additional temperature sensor. [9] describes a method to compensate the temperature dependence of the Faraday effect in interferometric sensors with non-reciprocal phase modulation. Here, the fiber-optic quarter-wave retarder that generates the circular light waves is prepared in a way that the temperature effect from the retarder balances the temperature effect from the Verdet constant.
K. Bohnert et al. in J. of Lightwave Technology, Vol. 20, No. 2, pp. 267-276 describes a device with non-reciprocal phase modulation, wherein temperature compensation in the non-reciprocal phase modulation scheme has been done by detuning the quarter wave retarder.
State-of-art interferometric fiber-optic current sensors with non-reciprocal phase modulation are high-end sensors with excellent accuracy for both alternating and direct currents. On the other hand they require sophisticated and expensive means to measure the magneto-optic phase shift, such as an integrated-optic phase modulator with closed-loop electronics. Moreover, the fiber connecting the opto-electronics module (comprising the light source, photo-detectors, signal processing electronics, etc) and the sensing fiber coil is a polarization-maintaining specialty fiber. Cables and connectors for such fibers are demanding and expensive.
The performance of simpler sensors with passive detection schemes is often not sufficient for applications in electric power transmission and distribution, particularly due to disturbing effects of temperature.